package correct.day16;

import java.util.Scanner;

public class Test33 {
    public static void main(String[] args)
    {
        Scanner in = new Scanner(System.in);
        char[] s = in.next().toCharArray();
        int x = in.nextInt();
        int begin = 0; // 标记结果的起始位置
        int maxCount = 0; // 统计以前窗⼝内 C + G 的最⼤值
        int left = 0, right = 0, n = s.length;
        int count = 0; // 统计窗⼝内 C + G
        while(right < n)
        {
            if(s[right] == 'C' || s[right] == 'G')
                count++;
            while(right - left + 1 > x)
            {
                if(s[left] == 'C' || s[left] == 'G') count--;
                left++;
            }
            if(right - left + 1 == x)
            {
                if(count > maxCount)
                {
                    begin = left;
                    maxCount = count;
                }
            }
            right++;
        }
        for(int i = begin; i < begin + x; i++)
        {
            System.out.print(s[i]);
        }
    }
}
